As a resistive random-access memory device that stores data using a change in the resistance of each element, there is disclosed a magnetic random-access memory (MRAM). As MRAM writing methods, there are known a current-field writing method and a spin-transfer torque (STT) writing method. Among these methods, the STT wiring method has a property that a spin-transfer current necessary for magnetization reversal is lower when a magnetic body is smaller in size. Therefore, the spin-transfer-torque wiring method is advantageous for realizing high integration, low power consumption, and high performance. The current-field writing method has a disadvantage in that potential false writes take place in unselected memory cells due to expansion of a magnetic field. By contrast, the STT writing method can avoid such false writes in unselected memory cells.
An STT MRAM is often configured to provide two selection transistors in each memory cell so as to write data at high speed by carrying a current through an MTJ (Magnetic Tunnel Junction) element of the memory cell. In the MRAM configured to provide two selection transistors per MTJ element, the layout density of MTJ elements locally differs in a memory cell array. If different MTJ-element layout densities are present, an irregularity increases in the size of the MTJ element depending on lithography and etching in a process of manufacturing MTJ elements. The irregularity in the size of the MTJ element leads to an irregularity in a signal amount at the time of reading data. This results in erroneous data detection and degraded reliability.